Capacitor measuring and detecting device



United States Patent 3,243,998 CAPACITOR NLEASURENG AND DETECTING DEVICERobert E. Vosteen, 5 Vernon St., Middleport, N.Y. Continuation ofapplication Ser. No. 66,274, Oct. 31, 1960. This application Nov. 17,1964, Ser. No. 414,938 3 Claims. (Cl. 73-141) This application is acontinuation of my copending application Serial No. 66,274, filedOctober 31, 1960, which claims the capacitor device itself.

This invention relates to an improved capacitor transducer device, andmore particularly, to such a device having particular application in themeasuring field, such as in measurement of a non-electrical property byelectrical means.

This invention meets a long-felt requirement for a single capacitordevice which is simple, structurally sound, and adaptable to a pluralityof useful purposes, such as measuring, detecting, and signalling, as,for example, in telemetering. The structure of the capacitor deviceincludes a metallic foil disposed parallel to and between a pair ofmetal plates, thereby forming a pair of capacitors, so that change inposition of the foil relative to the two plates can be utilized toderive a desired indication of a given parameter. Among the applicationsof the device is its use as a means for electrically detecting andmeasuring a change in pressure. In this application, it is possible tosubject opposite sides of the foil to different pressures, therebycausing displacement of the foil toward one or the other of thecapacitor plates. The change in capacities can thus be detected by anappropriate circuit, whereby the change in pressure can be detected ormeasured. A problem arises with this arrangement, however, in thatdeformation of the foil resulting from the differential pressure can bepermanent. This phenomena may be likened to canning, or the deformationof the ends of a can due to differential pressures within and withoutthe can. In the novel structure of this improved transducer, thisproblem is met and the undesirable canning is eliminated. A stillfurther problem, in structure of the type represented by thistransducer, is the effect of heat variation between the component parts,often resulting in deleterious effects upon the accurate performance ofthe device. This problem is also met by the teaching of this invention.

Within the circuitry of a suitable environment for the novel transducerof this invention there is provided a transformer having the secondarywinding thereof forming impedance elements of a bridge circuit. In thisarrangement, a high distributed capacitance exists between the finish ofthe primary winding and the start of the secondary winding relative tothe distributed capacitance between the start of the primary winding andthe finish of the secondary winding. This problem is also met within theteaching of this invention by a novel capacitance balance adjustment.Within the same circuitry, another problem exists due to leakage betweenthe primary and secondary windings or any leakage to ground, thisleakage resulting in an undesirable quadrature component of unbalance inthe bridge circuit. This problem is also met within the teaching of thisinvention by a novel resistance balance control.

The object of this invention is to provide an improved capacitortransducer device.

Another object is to provide an improved capacitor device utilizing athin metallic foil disposed parallel to and between capacitive plateswhereby a differential capacitance may be obtained.

Another object of this invention is to provide an improved capacitordevice which may be subjected to a ice differential force and which maybe readily utilized in a plurality of applications.

A still further object of the invention is to provide an improveddifferential capacitance transducer for electrically measuringnon-electric properties.

A still further object of this invention is to provide an electricaldetecting and measuring circuit incorporating a transformer, thesecondary winding of which forms impedance legs of a bridge circuit andwhich also incorporates a novel differential capacitance transducer.

A still further object of this invention is to provide, in a circuitincorporating a transformer having its secondary comprising impedancelegs of a bridge circuit, means for balancing an unbalance of capacitivedistribution and, with or without such a capacitive distribution balancein such a circuit, to provide a quadrature resistance balance betweenthe primary and secondary of the incorporated transformer.

Further objects, features, and attendant advantages of the inventionwill become apparent by reference to the following specification anddrawings, in which:

FIG. 1 is a schematic diagram of a detecting and measuring circuitembodying various features of the invention;

FIG. 2 is a sectional view taken along line 22 of FIG. 3; and

FIG. 3 is a side view of the differential capacitance transducer of apreferred embodiment of this invention.

With reference to FIG. 1, there is shown at 10 a source of highfrequency electrical energy. In the preferred embodiment of thiscircuit, this is an oscillator of suitable type. The output voltage fromoscillator 10 is applied across half of the primary winding of atransformer generally indicated at 20, and the entire primary is shuntedby a tuning capacitor 11. A first portion 22 of the primary winding oftransformer 20 extends between the start S and a center tap 23, while asecond portion 21 extends between center tap 23 and the finish F. Afirst portion 31 of the secondary of transformer 20 extends between thestart S and a center tap 33, while a second portion 32 of the secondaryextends between center tap 33 and the finish F.

A capacitor device, generally indicated at 40 has opposed capacitorplates 41 and 42. A thin, flexible metallic foil 43 is mounted parallelto and between plates 41 and 42. Foil 43 is grounded. A bridge circuit,generally indicated at 30, incorporates the secondary of transformer 20and the capacitance transducer 40. Specifically, the bridge comprisesportion 31 of the secondary of transformer 2t and in series therewith,the capacitance existing between plate 41 of transducer 40 and themetallic foil 43. In parallel with this leg is the second portion 32 ofthe secondary of transformer 20 and, in series therewith, thecapacitance formed by plate 42 and foil 43. Since the foil 43 isgrounded, it will be apparent that the output voltage of the bridge willappear between center tap 33 and ground. A wire 34 connects center tap33 to a pre-amplifier, generally indicated at 50. The output side ofpre-amplifier 5G is connected by a wire 51 to an attenuator, generallyindicated at 7% Wire 51 includes the secondary 62 of a transformergenerally indicated at 60. The primary 61 of transformer 60 isconnected, on one side, to ground, and on the other side, to the movabletap of potentiometer 12 which itself is connected across primary winding21. The output side of attenuator 70 is connected by a wire 71, througha blocking condenser 72, to an amplifier, generally indicated at 89.Attenuator output wire 71 is connected to the start S of the primary oftransformer 20. The output of amplifier is applied across the primarywinding 91 of a transformer, generally indicated at 90. The voltageappearing across secondary 92 of transformer is applied to a phasedetector, generally indicated at 169. The phase detector compares thebridge unbalance voltage from transformer 9%) with a reference voltagefrom oscillator 10, which is supplied through lead 13 connected tocenter tap 93 of the secondary 92 of the transformer.

Across the primary of transformer from start to finish thereof, there isconnected a potentiometer 24. The movable tap of the potentiometer isconnected through a resistor to the center tap 33 of the secondary oftransformer 20. A variabie'capacitor 26, and a fixed capacitor 27 inparallel therewith, are connected between the start of the primary oftransformer 20 and the finish ofthe secondary.

The operation of the circuit of FIG. 1 will now be explained for thecircumstance wherein differential pressure is the quantity or parameterbeing measured or detected. The high frequency output from oscillator 10is applied through transformer 20 to the bridge circuit 30. Differentialpressure acting upon opposite sides of the foil 43 of transducer 40causes a deflection of the foil toward one of the plates 41 or 42, andaway from the other. The resulting change in capacitance creates anunbalance in the bridge 30, resulting in a voltage of amplitudedetermined by the magnitude of the pressure difference, between centertap 33 and ground. This signal is applied to pre-amplifier which ispreferably of the transistor type and is designed to have a very highinput impedance to avoid loading the transducer impedance. The output ofthe pre-amplifier is supplied to attenuator through the secondary oftransformer 64 which provides a voltage which bucks out residualin-phase voltage unbalance from pre-ampliiier St). The attenuated signalis amplified by amplifier 81}, which feeds the signal throughtransformer to the phase detector 100. The detector furnishes a voltageof magnitude responsive to the magnitude of the bridge output and ofpolarity dependent upon whether the output is in phase or out of phasewith respect to the oscillator voltage. The amplitude of the phasedetector output voltage is indicated by meter ltll and is proportionalto the diflerential pressure which produced the bridge unbalance.

With particular reference to transformer 26, it will be understood thata high distributed capacitance will exist between the finish F of theprimary and the start S of the secondary, relative to distributedcapacitance between the start S of the primary and the finish" F of thesecondary. In order to compensate for the unbalance of distributedcapacitance, a capacitance balance or null balance adjustment isprovided. This adjustment comprises capacitors 26 and 27 which areconnected, in parallel, between the start S of the primary and thefinish F of the secondary. Capacitor 27 is fixed, while the adjustmentis provided by variation of capacitor 26.

Leakage between the primary and secondary of transformer 20 or anyleakage to ground, disproportionate as between primary and secondary,will result in a quadrature component of unbalance in the bridge output.To correct this undesirable result of leakage, a portion of the primaryvoltage selected by the movable tap of potentiometer 24 is connectedacross the bridge output.

It has been pointed out that transformer 60 provides a Ibuclcing voltagebucking out residual in-phase or 180 out-of-phase bridge unbalance frompre-amplifier 50. In similar manner, the connection of the outputcircuit of the oscillator 10 to wire 71 provides a bucking voltageagainst undesirable quadrature voltage components induced in attenuator70.

Having described a measuring and detecting device incorporating thetransducer of the invention, as well as and foil 43 are provided withholes to receive fastening means, such as bolts 47, by which the flangesare drawn together in tight abutting relationship. Since the foilextends from one periphery to the other of the flanges 44, the outerperipheries of flanges 44 define the outer cylindrical con-tour of thetransducer, while the inner peripheries define the cylindrical wallhousing the transducer device. End walls for the cylindrical housing areprovided by covers 45 closing the opposite axial ends of the cylindricalhousing. Retained between coinciding grooves in the abutting radialfaces of flanges 44 is an O-ring 46 of compressible material.

Foil 43 extends between the outer peripheries of flanges 44. By theassembly of the flanges 44 intoabutting relationship with the foil 43and ring 46 disposed therebetween, foil 43 lies alongside ring 46 and iscaused to be deformed so as to pass around ring 46; Flanges 44 and 43are provided with holes to receive fastening means, such as bolts 47, bywhich the flanges are drawn together in tight abutting relationship.Since the foil extends from one periphery to the other of the flanges44, the force exerted normal to the plane of the-foil 43 and ring 46,the'latter of which is slightly deformable, results in the placing ofthe foil 43 under tension. The tensile force thus produced in foil 43causes it to return to its symmetrical plane coextensive with theabutting faces of flanges 44 after each deformation of the foil causedby differential pressure between the opposite sides thereof.

To assist in the exertion of force upon foil 43 through compression ofring 46 against it, it is preferable to form V the annular groove in oneof the flanges 44 of square U-shaped' cross section, and to form theannular groove in the face of the other flange 44 of V-shaped crosssection. The foil preferably lies onthe side of ring 46 within theV-shaped groove. Thus, by means of the walls of the V-shaped groove, thefoil is pressed upon ring 46 by force substantially perpendicular to atangent thereto, thus avoiding penetration of the foil by the sharpcorner of the U-shaped groove.

As best seen in FIG. 3, fastening means 47 which draws the adjacentflanges 44 tightly together, can be located about the cylindricalcircumference of the flange body, so as to exert uniform pressurethroughout the circumference of ring 46 and thus equal tensionthroughout foil 43.

Attached to condenser plates 41 and 42 are shafts 48 which extendthrough bushings 43' in covers 45. Shafts 4S and bushings 48' arecooperatively threaded by an extremely fine, micrometer thread. Theremote or outer ends of shafts 48 are provided with handles 49. Thus, bymeans of handles 49, either of the plates 41 and 42 may be adjustedtoward and away from foil 43 in a fine adjustment.

In capacitance devices of this type, and especially in the configurationheretofore described, it will be apparent that unequal expansion of anyof the parts relative to the other parts will result in deleteriouseffects upon the accurate performance of the transducer. For example,considering that a fine micrometer adjustment of theposition of plate 41or 42 relative to foil 43 may be made, any uneven expansion ordeformation of a cover 45, which carries the shaft 48, would result indisplacement of plate 41 or 42 within the capacitance space. To obviatesuch problems, the materials comprising the constituent parts are chosenso as to have a substantially identical temperature coefficient. Flanges44, foil 43, plates 41 and 42, and shafts 48 and 48 are preferablyconstructed of aluminum. Covers 45 are constructed of a. phenolic resinhaving a temperature coefficient substantially equal to that ofaluminum. Ring 46, being symmetrical, will be equally affected axiallyand about its entire circumference, so that its temperature coefficientwill not be critical within reasonable limits. Thus, heat to which thecapacitor parts are subjected will cause no uneven or asymmetricalexpansion of the parts with consequent inaccuracy in the detection,measuring or other function of the transducer.

For adaptation of the transducer of this invention to sense differentialpressure, as described in connection with FIG. 1, the interiorcapacitance space is subjected to pressure different on one side of thefoil from that on the other. For that purpose, a passage 2%1 is providedthrough one of the flanges 44 so as to enter the capacitor space withinthe flange. Connecting with passage 201 is a suitable pipe or conduit262, which may be connected to any desired source of pressure to bemeasured or detected. As best shown in FIG. 3, a pipe or conduit 2%connects with a passage, not shown, entering the space opposite the foil43 from that with which passage 2G1 connects.

While the adaptation of the transducer has been set forth in theparticular detecting device of FIG. 1, it should be understood that thetransducer may have utility in a number of other and variedapplications. For example, since the foil 43 is grounded, it is possibleto apply, to one of the capacitor plates 41 or 42, a high frequencyvoltage, and to the other plate a high frequency voltage of either thesame or different frequency. Across the terminals of the transducer,then, may be taken a difference frequency which may be transmitted invarious applications having great utility, such as in the art oftelemetering.

From the foregoing, it is apparent that the invention thus describedprovides a transducer which is rugged, simple, reliable, and adaptableto a number of useful applications. When incorporated in a bridgecircuit of the type described, it can be used to sense differentialpressure and thus indicate change in pressure by generating a signalresponsive to unbalance in the bridge circuit. In this use of thetransducer, various problems arise in adapting the transducer to theparticular circuit; however, these problems have been overcome by thenovel circuit features described above.

Although the foregoing embodiment has been illustrated and described, itis apparent to those skilled in the 6 art that various changes andmodifications may be made in the construction arrangements of thevarious parts Without departing from the scope of the invention; Itherefore do not wish to be limited to the specific embodiments shown orsuggested, but only by the scope of the limitations which follow.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A measuring device comprising a source of high frequency electricalenergy, a transformer having a primary windin and a center-tappedsecondary Winding, said primary winding being connected to said sourceto receive high frequency current therefrom, a capacitor device havingtwo plates and a grounded foil disposed between said plates and subjectto deflection in response to a force being measured, said secondaryWinding and said capacitor device connected together to form legs of abridge circuit, amplifier means connected between said secondarycenter-tap and ground operable to supply a voltage indicative of themagnitude of said force, a potentiometer connected across the primary ofsaid transformer, and means connecting the movable tap of saidpotentiometer to the center-tap of said secondary winding.

2. The apparatus of claim 1 including a variable capacitor connectedbetween the primary and secondary of said transformer at the start andfinish portions thereof, respectively.

3. The apparatus of calim 1 including a center-tap on said primaryWinding of said transformer, a second potentiometer connected acrosssaid primary winding, and means including a transformer having itsprimary winding connected between said primary center-tap and themovable tap of said second potentiometer for supplying balancing currentto the output of said amplifier means.

No references cited.

RICHARD C. QUEISSER, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,243,998 April 5, 1966 Robert E. Vosteen It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 4 line 1 beginning with "and foil" strike out all to andincluding "44, the" in line 4, and insert instead and 42 there is a thinmetallic web or foil 43. A pair of annular flanges 44 is arranged withthe adjacent radial faces of the flanges in abutment with one another.The column 6, line 28, for "calim" read claim Signed and sealed this26th day of September 1967.

(SEAL) Attest: ERNEST w. SWIDER EDWARD J. BRENNER Attesting OfficerCommissioner of Patents

1. A MEASURING DEVICE COMPRISING A SOURCE OF HIGH FREQUENCY ELECTRICALENERGY, A TRANSFORMER HAVING A PRIMARY WINDING AND A CENTER-TAPPEDSECONDARY WINDING, SAID PRIMARY WINDING BEING CONNECTED TO SAID SOURCETO RECEIVE HIGH FREQUENCY CURRENT THEREFROM, A CAPACITOR DEVICE HAVINGTWO PLATES AND A GROUNDED FOIL DISPOSED BETWEEN SAID PLATES AND SUBJECTTO DEFLECTION IN RESPONSE TO A FORCE BEING MEASURED, SAID SECONDARYWINDING AND SAID CAPACITOR DEVICE CONNECTED TOGETHER TO FORM LEGS OF ABRIDGE CIRCUIT, AMPLIFIER MEANS CONNECTED BETWEEN SAID SECONDARYCENTER-TAP AND GROUND OPERABLE TO SUPPLY A VOLTAGE INDICATIVE OF THEMAGNITUDE OF SAID FORCE, A POTENTIOMETER CONNECTED ACROSS THE PRIMARY OFSAID TRANSFORMER, AND MEANS CONNECTING THE MOVABLE TAP OF SAIDPOTENTIOMETER TO THE CENTER-TAP OF SAID SECONDARY WINDING.